1. Field of the Invention:
The present invention relates to a display device which consumes less electric power and provides high brightness on a screen. In particular, the present invention relates to improvement in a display device for displaying a picture on an optical waveguide plate corresponding to an image signal by controlling displacement movement of actuator elements in a direction to make contact or separation with respect to the optical waveguide plate in accordance with an attribute of the inputted image signal to control leakage light at predetermined positions on the optical waveguide plate.
2. Description of the Related Art:
Display devices such as cathode-ray tube (CRT) and liquid crystal display devices have been hitherto known.
Those known as the cathode-ray tube include, for example, ordinary television sets and monitor devices for computers. Although the cathode-ray tube provides a bright screen, it involves problems in that a large amount of electric power is consumed, and an entire display device has a large depth or length as compared with a size of the screen.
On the other hand, the liquid crystal display device is advantageous in that an entire device can be miniaturized, and the amount of electric power consumption is small. However, the liquid crystal display device involves problems in that the screen brightness is inferior, and the screen has a narrow angle of view.
Further, when a color screen is produced by using the cathode-ray tube or the liquid crystal display device, it is necessary that the number of picture elements is three times that of a black-and-white screen. For this reason, problems also arise in that the device itself is complicated, the electric power consumption increases, and the cost inevitably increases.
In order to solve the foregoing problems, the present applicant has been suggested a novel display device (for example, see Japanese Laid-Open Patent Publication No. 7-287176). As shown in FIG. 30, this display device includes actuator elements 100 arranged for each of picture elements. Each of the actuator elements 100 comprises a main actuator element 108 and a substrate 114. The main actuator element 108 includes a piezoelectric/electrostrictive layer 102 and upper lower electrodes 104, 106 formed on upper and lower surfaces of the piezoelectric/electrostrictive layer 102 respectively. The substrate 114, which is disposed under the main actuator element 108, includes a vibrating section 110 and a fixed section 112. The lower electrode 106 of the main actuator element 108 contacts with the vibrating section 110. The main actuator body 108 is supported by the vibrating section 110.
The substrate 114, which is composed of a ceramic material, is constructed by integrating the vibrating section 110 and the fixed section 112. A recess 116 is formed in the substrate 114 so that the vibrating section 110 is thin-walled.
A displacement-transmitting section 120 for ensuring a predetermined size of a contact area with respect to an optical waveguide plate 118 is connected to the upper electrode 104 of the main actuator element 108. In the illustrative device shown in FIG. 30, the displacement-transmitting section 120 is disposed closely to the optical waveguide plate 118 in a state of OFF selection or NO selection in which the actuator element 100 stands still, while in a state of ON selection, the displacement-transmitting section 120 is disposed to contact with the optical waveguide plate 118 with a distance of not more than the wavelength of light intervening therebetween, if any.
Light 122 is introduced, for example, from an end of the optical waveguide plate 118. In this arrangement, all of the light 122 is subjected to total reflection at the inside of the optical waveguide plate 118 without being transmitted through front and back surfaces of the optical waveguide plate 118, by controlling the magnitude of the refractive index of the optical waveguide plate 118. In this state, a voltage signal corresponding to an attribute of an image signal is selectively applied to the actuator element 100 via the upper electrode 104 and the lower electrode 106 to allow the actuator element 100 to perform various displacement movements based on ON selection, OFF selection, and NO selection. Thus the displacement-transmitting section 120 is controlled for its contact and separation with respect to the optical waveguide plate 118. Accordingly, scattered light (leakage light) 124 is controlled at a predetermined position of the optical waveguide plate 118, and a picture is displayed on the optical waveguide plate 118 in accordance with the image signal.
This display device is advantageous, for example, in that (1) the electric power consumption can be decreased, (2) the screen brightness can be increased, and (3) it is unnecessary to increase the number of picture elements when a color screen is produced, as compared with a black-and-white screen.
However, the illustrative display device suggested by the applicant has a so-called sandwich structure of the main actuator element 108 in which the upper electrode 104 and the lower electrode 106 are formed on the piezoelectric/electrostrictive layer 102. Therefore, it is feared that the electrostatic capacity of the main actuator element 108 may inevitably become large, and the CR time constant for signal transmittance may become large in view of wiring resistance between the mutual main actuator elements 108.
If the CR time constant becomes large, a problem arises in that rounding occurs in signal waveform of a voltage signal corresponding to an attribute of an image signal, and it is impossible to apply a specified voltage to each of the electrodes. This results in failure in provision of necessary distortion to the piezoelectric/electrostrictive layer 102. Especially, it is feared that the display brightness is weakened at portions corresponding to the actuator elements 100 arranged at positions (for example, peripheral and central portions of the screen) far from the voltage signal supply point.
In the case of the illustrative display device suggested by the applicant, the actuator element 100, which is composed of the main actuator element 108 having the sandwich structure, the vibrating section 110, and the fixed section, has a certain bending displacement characteristic as shown in FIG. 31B. Namely, the bending displacement characteristic is symmetrical in positive and negative directions of the electric field in relation to a reference electric field point (point of the electric field=0) as a center. It is assumed that the direction of the bending displacement is positive when the actuator element 100 is displaced in a convex manner in a first direction (direction for the upper electrode 104 formed on the piezoelectric/electrostrictive layer 102 to face the free space), while the direction of the bending displacement is negative when the actuator element 100 is displaced in a concave manner.
The displacement characteristic is obtained by observing the displacement of the actuator element 100 as follows. Namely, the piezoelectric/electrostrictive layer 102 is subjected to a polarization treatment by applying a predetermined voltage between the upper electrode 104 and the lower electrode 106. After that, the voltage applied between the upper electrode 104 and the lower electrode 106 is continuously changed so that the electric field applied to the actuator element 100 changes to, for example, electric fields of +3E.fwdarw.-3E.fwdarw.+3E.
Namely, at first, an electric field for polarization (for example, +5E) is applied in the positive direction to the actuator element 100 to perform the polarization treatment for the piezoelectric/electrostrictive layer 10. After that, the voltage application between the upper electrode 104 and the lower electrode 106 is stopped to give a no-voltage-loaded state. Simultaneously with the start of measurement, a sine wave having a frequency of 1 Hz and peak values of .+-.3E (see FIG. 31A) is applied to the actuator element 100. During this process, the displacement amount is continuously measured at respective points (Point A to Point D) by using a laser displacement meter. FIG. 31B shows a characteristic curve obtained by plotting results of the measurement on a graph of electric field-bending displacement. As indicated by arrows in FIG. 31B, the displacement amount of the bending displacement continuously changes in accordance with continuous increase and decrease in electric field.
Specifically, it is assumed that the measurement is started from an electric field +3E. At first, as shown in FIG. 32A, the electric field is applied to the actuator element 100 in the same direction as that of the polarization direction. Accordingly, the piezoelectric/electrostrictive layer 102 is elongated in a direction across the upper electrode 104 and the lower electrode 106, and it is contracted in a direction parallel to the upper electrode 104 and the lower electrode 106. As a result, the entire actuator element is displaced in the negative direction in an amount of about 0.9 .DELTA.y.
After that, when the electric field is changed from +3E to -0.5E, the displacement amount is gradually decreased. When the electric field is in the negative direction, as shown in FIG. 32B, the electric field is applied in the direction opposite to the polarization direction. Therefore, elongation occurs in the piezoelectric/electrostrictive layer 102 in the direction parallel to the upper electrode 104 and the lower electrode 106, and the displacement is changed to the positive direction.
Next, when the electric field is changed in a direction of -0.5E.fwdarw.-3E, the polarization direction is gradually inverted. Namely, the direction of the electric field is gradually aligned with the polarization direction. As for Point B.fwdarw.Point c.fwdarw.Point C in FIG. 31B, it is assumed that the polarization is inverted approximately completely at Point c, because no hysteresis is observed between Point c and Point C.
As shown in FIG. 33A, the alignment of the direction of the electric field with the polarization direction allows the piezoelectric/electrostrictive layer 102 to change from the state of horizontal elongation to a state of contraction. At a stage at which the electric field is -3E, the displacement amount is approximately the same as the displacement amount (0.9 .DELTA.y) obtained at the start point of the measurement.
Namely, when the polarization direction is coincident with the direction of the electric field, the piezoelectric/electrostrictive layer 102 is contracted in the direction parallel to the electrodes 104, 106 (elongated in the direction across the electrodes 104, 106). This situation corresponds to the states represented by Point A and Point C. When the polarization direction is opposite to the direction of the electric field, the piezoelectric/electrostrictive layer 102 is elongated in the direction parallel to the electrodes 104, 106 (contracted in the direction across the electrodes 104, 106). This situation corresponds to the states represented by Point B and Point D. It is noted that there are given 1E=about 1.7 kV/mm and 1 .DELTA.y=about 1.6 .mu.m.
After that, when the electric field is changed from -3E to +0.5E, the displacement amount is gradually decreased. When the electric field is in the positive direction, as shown in FIG. 33B, the electric field is applied in the direction opposite to the polarization direction. Accordingly, elongation occurs in the piezoelectric/electrostrictive layer 102 in the direction parallel to the upper electrode 104 and the lower electrode 106, and the displacement is changed to the positive direction.
When the electric field is changed in a direction of +0.5E.fwdarw.+3E, the polarization direction is gradually inverted. When the direction of the electric field is aligned with the polarization direction, the piezoelectric/electrostrictive layer 102 is changed from the state of horizontal elongation to a state of contraction.
As described above, in the case of the actuator element 100 of the illustrative display device suggested by the applicant, the bending displacement characteristic is symmetrical in the positive and negative directions in relation to the reference electric field point (electric field E=0) as the center. Therefore, the relative displacement amount is small between the no-voltage-loaded state and the voltage-applied state, and the relative displacement amount is small between the states in which mutually opposite electric fields are applied respectively. As a result, it is feared that the control of the actuator element 100 may become difficult. This fact involves a possibility for the display device to be disadvantageous from a viewpoint of improvement in image quality. Accordingly, it is necessary to promptly make a countermeasure thereagainst.